Add a new keywork xmlcatmgr
It simplifies the handling of the XML and SMGL catalog
It brings a big of consistency by always specifying the catalog path absolute
instead of mixing absolute and relative path.
The keyword is also written a PKG_ROOTDIR friendly to simplify cross installing
Reviewed by: hrs
Differential Revision: https://reviews.freebsd.org/D6539

textproc/docbook-xml: explicitly use ${TAR} for extraction
Despite all 6 distribution files being zip archives, the extraction target
is using a tar-specific set of arguments. USES=zip cannot be used (native
or infozip) so explicitly require ${TAR} for future-proofing reasons.
Approved by: Infrastructure blanket

Fix catalog generation to unbreak textproc/docbook-utils build.
Missing closing quotes, and docbook-xml-5.0 does not install a
docbook.cat for SGML, so do not list it. Use a shell test to skip the
installation because fmake would otherwise barf (bmake is fine).

Partially revert r284943 to enable building with base unzip.
The better solution would be to do the extraction in the do-extract section.
PR: ports/182648
Submitted by: ports/182022
Approved by: maintainer timeout (kuriyama, 4 weeks)

Fix DISTFILE fennerage. I'm only the MAINTAINER for docbook-310, but the
other docbook-* ports were broken in the same way, so I've taken the liberty
of updating the URL for all of them -- it looks as though OASIS have fallen
over, so point these at docbook.org for the time being.